1. Field of the Invention
The present invention relates to an apparatus for forming a thin film by evaporation on a substrate by cluster ion beam evaporation technique and also to a method of forming a thin film.
2. Description of the Related Art
FIG. 4 is a schematic sectional view of a known thin film forming apparatus disclosed, for example, in Japanese Patent Publication No. 54-9592, while FIG. 5 is a partly-sectioned perspective view of the apparatus shown in FIG. 4.
Referring to these Figures, a vacuum exhausting device (not shown) is connected to a discharge passage 2 annexed to a vacuum tank 1. A bed plate 4 is fixed to the bottom of the vacuum tank 1 through a plurality of pillars 3. A crucible 8 is mounted on the bed plate 4 through the intermediary of a support bar 5, insulating support member 6 and a crucible support 7. The crucible 8 is charged with a material 9 which is to be evaporated and deposited, e.g., copper (Cu). One or more nozzles 8b, each having a diameter of 1 to 2 mm, are connected to a lid 8a of the crucible 8.
A heating filament 10 for heating the crucible 8 surrounds the latter. A heat shield plate 11 is provided around the filament 10. The crucible 8, heating filament 10, heat shield plate 11 and a power supply (not shown) in cooperation form an evaporation source 12 which forms a cluster beam 27.
A thermal electron emitting portion 13, which emits ionizing thermal electrons 14, is disposed above the evaporation source 12. The thermal electron emitting portion 13 includes an ionizing filament 15, and an electron extracting electrode 16 which extracts thermal electrons 14 from the ionizing filament 15 and applies the extracted thermal electrons 14 to a cluster beam 27. A heat shielding plate 17 for shielding radiated heat is disposed around the filament 15. The thermal electron emitting portion 13, a power supply (not shown) for the thermal electron emitting portion 13, and the heat shielding plate 17 in cooperation form ionizing means 18 which ionizes the cluster beam 27 to generate cluster ions 28.
An accelerating electrode 19 disposed above the ionizing means 18 is adapted to be activated by an accelerating power supply (not shown) so as to establish a potential difference between itself and the electrode 16. Accelerating means 30 is composed of the accelerating electrode 19 and the accelerating power supply.
The heat shielding plates 11, 17, the electron extraction electrode 16 and the acceleration electrode 19 are supported in an insulated manner by an insulating support member 20 which in turn is supported on the bed plate 4 through a supporting post 21 and an insulator 22. A substrate 23 is carried by a substrate holder 24 which is secured to the top of the vacuum vessel 1 through insulators 25. Thus, the substrate is disposed at upper portion of the space inside the vacuum vessel 1.
In the use of this known thin film forming apparatus, the crucible 8 is charged with the material 9 to be evaporated, and the interior of the vacuum vessel 1 is evacuated to a high degree of vacuum, e.g., 10.sup.-4 Pa or so(about 10.sup.-6 Torr). Subsequently, the heating filament 10 is energized to generate heat, so that the crucible 8 is heated by collision with thermal electrons and by radiation heat, whereby the evaporation material 9 is evaporated. The heating of the crucible 8 is conducted to such an extent that the vapor pressure of the evaporation material of about 10 to 10.sup.3 Pa (0.1 to several tens of Torr.). In this state, due to adiabatic expansion caused by the difference in the pressure between interior and exterior of the crucible 8, many atoms are loosely aggregated to form a bulk of atoms, i.e., clusters 26, thus producing the cluster beam 27.
The cluster beam 27 is irradiated with thermal electrons 14 emitted from the ionizing filament 15. Consequently, an atom in a certain cluster in the cluster beam 27 is ionized to form a cluster ion 28. The cluster ion 28 thus formed is moderately accelerated by the electric field formed between the accelerating electrode 19 and the electron extracting electrode 16, and collides with the substrate 23 together with non-ionized neutral cluster 6, whereby a thin film is deposited on the substrate 23.
In the known method for ionizing the cluster beam 27, the ionizing current produced by the thermal electrons 14 is a D.C. current and is kept constant during evaporation. Therefore, the amount of the cluster ions on the substrate 3 is determined by the level of the ionizing current which is initially set when the evaporation deposition is commenced.
FIG. 6 is a diagram showing the relationship between substrate radius and the cluster ion current density, illustrative of an example of ion current density as measured on the substrate 23 when the evaporation deposition was conducted by the known ionizing method. The evaporation deposition was conducted by using copper as the evaporation material 9 at an evaporation deposition rate of 24 nm/min and accelerating current of 1.5 kV, while varying the level of the ionizing current. The measurement of the cluster ion current was conducted by moving a Faraday cup of an aperture size of 3.6 mm within a horizontal plane 400 mm above the crucible 8. From the current density distribution of the cluster ions on the substrate 23, it is understood that the cluster ions 28 tend to concentrate to the central region of the substrate 23 when the level of the ionizing current is small and that cluster ions 28 on the peripheral portion of the substrate 23 increase as the level of the ionizing current becomes higher.
Thus, in the described known thin film forming apparatus of the present invention, there is no means for varying the amount of cluster ions on the central region of the substrate, although the amount of cluster ions on the peripheral region of the same is controllable by changing the level of the ionizing current. It has been difficult to adequately control the characteristics of the thin film such as affinity of the film to the substrate and crystallinity of the same by a control of the amount of the cluster ions, particularly when the substrate 23 used has a large area.